Optical detectors are important to a variety of laboratory and industrial techniques. One example of such a detector is an array of optically sensitive elements forming its pixels, such as an array of optically responsive charge coupled devices. In typical applications, the output of the array is the sum of the responses (the total charge accumulated by all the pixels) in response to a given illumination. Unfortunately, no two charge coupled devices (or any other device which can be used as an optical detector) is perfectly linear, nor can any two such devices be manufactured identical to one another. These imperfections affect the overall response of the array, more markedly so as the features of the image on the detector becomes small with respect to the area of the array exposed to illumination. Thus to be useful, such a device must be calibrated accurately as a function of spatial frequency.
Prior techniques for performing this calibration have not been wholly satisfactory, particularly at higher spatial frequencies. One approach has been to use a sharp, opaque, surface combined with lenses and other optics to spatially localize illumination on a selected part of the array. One measures the array's response, and then proceeds to another portion of the array. Finally, all the individual responses are integrated to infer the device's overall response to the input light. This response, expressed as a function of spatial frequency, is often referred to as a Modulation Transfer Function, or MTF, of the device, and is the ratio of the output signal which device 18 produces to the optical signal input, as a function of spatial frequency. Unfortunately, this approach necessarily limits the size of the image to one much smaller than the array itself, which, among other effects, scatters energy into higher spatial frequencies, which introduces inaccuracies when attempting to account for all the energy transduced by the array so as to calculate the MTF. Similarly, the edge itself and the lenses perturb the optical wavefront of the input illumination, which also has energy at higher spatial frequencies. It is difficult to correct for all these effects of the optics used to focus the illumination, especially at higher spatial frequencies. Moreover, the lenses focus the incident image onto a focal plane which typically has a narrow depth of focus, limited area of extent, and has a curvature that rarely matches that of the object's surface. This makes the placement of the object with respect to the lenses both critical and difficult.
Another approach is to use lenses to image a bar pattern, or pattern of sinusoidally varying translucence, onto the array, and measure its response. The response provides one point of the MTF, i.e. that of the spatial frequency corresponding to that of the bar or sinusoidally varying pattern. One generates the entire MTF by repeating this measurement for all spatial frequencies of interest. Unfortunately, the fidelity of this measurement is again compromised by the lenses' effects and the finite extent of the image produced, by the lenses limited depth of focus, as well as by the limits with which one can accurately produce bar or sinusoidally varying patterns in the first place, particularly at higher spatial frequencies.